All 15 participant images were from a human ethics approved bladder cancer protocol,
full details of this protocol have been previously published15.Patients underwent CT simulation using 3mm thick slices by 3mm spacing,
after emptying their bladder completely. Immobilisation was with ankle stocks and a bolster under the knees. A single radiation oncologist (FF) completed contouring of the bladder,
primary tumour,
rectum,
and non-rectal bowel prior to commencement of the project.
All 3D-CRT,
IMRT and VMAT plans were created prospectively using the unique set of contours for each patient by one experienced radiation therapist investigator (LW).
A standalone Eclipse (Varian,Palo Alto,CA) treatment planning system,
installed on a Dell Precision T5500 computer,
was used for all plan creation.
The radiation therapist had extensive experience with 3D-CRT and IMRT planning,
as well as training but lesser experience with VMAT planning.
The treatment plans were based on whole bladder treatment with a single phase treatment.
The study was based on the original planning CT and contoured volumes in each of the 15 patients. The CTV consisted of the gross tumour volume (GTV) and the whole bladder. Pelvic lymph nodes were not an elective part of the volume in any patient. The CTV to PTV margin was 1.5cm. Non rectal bowel and rectum were contoured 4 slices (12mm) above and below the PTV.
Each patient had one 3D CRT,
one IMRT and one VMAT plan created by the same radiation therapist (LW). The same dose constraints were used for creation of 3D CRT,
IMRT and VMAT plans (table 1)
|
3D-CRT/IMRT/VMAT |
PTV |
D2%≥60.8Gy
D98%≤ 68.5Gy
Median absorbed dose (D50%) to be ±2% of 64Gy in 32 fractions of 2Gy each |
Bladder |
V15≤80%
V25≤75%
V35≤70%
V50≤65% |
Rectum |
D50≤50Gy
D40≤64Gy
V50 < 50%,
V60 < 35%,
V65 < 25%,
V70 < 20%,
V75 <15%
|
Rt Fem Head and Lt Fem Head |
D100≤ 35Gy
D60≤45Gy
D10≤50Gy |
Small bowel (non rectal) |
V15Gy≤350cc
V30Gy ≤ 150cc
V35Gy ≤ 120cc
V45Gy ≤ 50 cc
V50Gy = 0 cc |
Table 1 Clinical Dosimetric Constraints used in Planning
The total prescription dose to the PTV was based on a median dose of 64Gy delivered in 32 fractions at 2Gy per fraction treating daily,
five days per week over six and the half weeks.
Plans were normalised to ensure that the 95% isodose adequately covered the PTV and that the dose distribution was such that the minimum dose to 99% of the PTV (D99%) was greater than or equal to 95% of the prescribed dose (60.8Gy) and the mean CTV dose was within 0.5Gy of the prescribed dose.
Raw dosimetry was also collected.
All plans were evaluated to ensure they met our institutional dose constraints outlined in table 1.
All 3DCRT plans were composed of a 5 beam field arrangement utilising 18MV photons.
MLC beam shaping and beam modifiers (eg wedges) were employed as required to produce the most conformal dosimetry.
All IMRT and VMAT plans were created using a 6MV photon beam applicable to a Varian Clinac iX (Varian Medical Systems,
Palo Alto,
CA) linear accelerator with a 120 leaf Millennium dynamic multileaf collimator (MLC).
Simultaneous infield boost (SIB)
10 of the 15 patients whom had well localised tumours were considered for a SIB.
Patients with multiple tumours were excluded from SIB planning.
For the SIB the bladder tumour/site of disease was contoured as CTV2 and a 2 cm margin used to create PTV2. The SIB dose was an additional 10Gy resulting to a total dose of 74Gy in 32 fractions.
IMRT Plans
IMRT plans were generated on the Eclipse Version 8.9.08 treatment planning software (Varian Medical Systems,
Palo Alto,
CA) using a 7 beam multifield technique.
The initial optimisation parameters and their priorities were set according to our institutional optimisation protocol and then adjusted as required to achieve the dose constraints (table 1) using a minimum of 60 iterations.
A Normal Tissue Objective was included and the default smoothing parameters were applied to help reduce hotspots outside the PTV and the total monitor units.
VMAT Plans
VMAT plans utilising the Varian RapidArc technique (Varian Medical Systems,
Palo Alto,
CA) were planned using Eclipse Version 8.9.08 treatment planning software using the same CT-dataset and contoured volumes as the IMRT plans.
A single arc technique was used with the gantry set to rotate through 340o in a clockwise direction from a starting position of 190o to a final position of 170o to reduce the amount of treatment through the rectum and central rail of the couch top.
The collimator rotation was individually optimised for each patient but generally set at 45o to reduce the effect of tongue and groove leakage.
The final dose calculation for all 3 of the plans for each patient was performed using the anisotropic analytical algorithm (AAA) version 8.9.08 with a 2.5mm dose calculation grid space.
Delivery Times
Treatment was delivered for each of the 15 3D-CRT,
IMRT and VMAT plans using a Varian 21iX Linear Accelerator (Varian Medical Systems,
Palo Alto,
CA). All beams were delivered to an empty bunker.
Beam delivery time was measured using a stopwatch from the beginning of the first beam to the end of the last beam including all gantry movements.
Beam order was optimised to simulate live treatment scheduling. As no patient was involved in this process,
these treatment times do not include patient set-up time,
which would be expected to be the same for all three plans.